Measurement of large diameter cylindrical cases has herebefore been accomplished by essentially manual techniques which while adequate for construction type activities do not meet the needs for assembling together large segmented cylindrical cases of solid rocket motors. A need exists for continuous profile information of the cylindrical surfaces of adjacent segments that were to be joined together. The out-of-roundness of a rocket motor case segment is not necessarily objectionable if the adjacent case segments that were to be joined were similarly out-of-round. Manually performing measurements to support such a profile for a rocket motor case segment would literally run into thousands of measurements and have a significant possibility of human error. While apparatus for simply measuring the diameter of an internal cylindrical surface has been accomplished, as shown by U.S. Pat. No. 4,521,968, with a radius arm with a linear transducer, such apparatus depends on having the rotating radius arm at the precise axis center of the cylinder that is defined by the cylindrical surface.